Cathode assembly for electron tube



A ril 9, 1963 H. v. KNAUF, JR

CATHODE ASSEMBLY FOR ELECTRON TUBE Filed March 14, 1960 2 Sheets-Sheet 1 INVENTOR. 62/73! M 471411], J1: By 77% 4 II'IIIIIIIIII.

Aprll 9, 1963 H. v. KNAUF, JR'

CATHODE ASSEMBLY FOR ELECTRON TUBE Filed March 14, 1960 2 Sheets-Sheet 2 INVENTOR. Harrq V. Klmuf Ir.

3,085,175 CATHGDE ASSEMBLY FOR ELECTRQN TUBE Harry V. Knauf, In, ll lonntainside, N1, assignor to Radio Corporation of America, a corporation of Delaware Filed Mar. 14, 1966, Ser. No. 14,958 9 Claims. (Cl. 313-270) This invention relates to improved cathode assemblies for electron tubes and to a method for making such cathode assemblies.

One type of presently available electron tubes includes a plurality of concentric electrodes supported on leadins and support conductors sealed through a ceramic disk header wafer. In the fabrication of such a tube the electrodes and lead-ins are assembled in a jig in desired strain-free contacting relationship and then brazed together into a rigid electrode mount assembly. .A cuplike envelope member, which may be of metal or ceramic, is disposed over the electrode mount assembly and sealed to the periphery of the header after tube bake-out and exhaust to form a vacuum-tight envelope. However, the cathode of such a tube is of the indirectly heated type and includes an electron emissive coating thereon. Inasmuch as this emissive coating cannot withstand the high temperatures encountered during the brazing step, the cathode is added after brazing of the rest of the mount assembly and before the envelope is put in place.

The cathode assembly of such an electron tube comprises a telescoped assembly of two tubular or sleeve parts. A first tubular member serves as the support sleeve and is part of the brazed assembly. A second tubular member closed at one end and providing the cathode is telescoped over the support sleeve after the mount assembly has been brazed together. This second member is coated with electron emissive material on the external surface thereof. The envelope is then placed over the mount assembly and in contact with the header. During the subsequent heating and exhaust processing the support and cathode sleeve are permanently sintered together. Sealing of the envelope is performed after bake-out and exhaust.

During the evacuation and sealing processing, the tube may be oriented in an inverted position. Because of size variations encountered in mass production of the support and cathode sleeves, the sleeves are not always of such size as to insure tight frictional engagement to maintain them in secure telescoped contact until sintered together. As a result, often times before actual sintering together of the sleeves take place, the cathode sleeve will fall off of its inner support sleeve unless it is fixed securely thereto. Welding together of the two sleeves is not suitable because their contact may be too slight to enable a good weld to be made. Furthermore, when sintering does take place, it frequently occurs only at a few spaced spots or along a single line thus resulting in small area contact. Such small area contact not only results in inefficient heat transfer from the support sleeve to the cathode sleeve in normal tube operation, but also affects the electrical resistance in the cathode circuit.

It is therefore an object of my invention to provide a new and improved indirectly heated cathode assembly structure which avoids or overcomes the problems of prior art structures set forth above.

It is another object of my invention to provide a novel and improved two-piece tubular telescoped cathode assembly structure, the parts of which can be manufactured with ordinary mass production tolerances and yet which can be telescoped and consistantly provide relatively large area interference fitting contact substantially uniform from cathode to cathode and which promotes uniformity 3,685,175 Patented Apr. 9, 1963 in the electrical characteristics of the electron discharge devices in which they are used.

Briefly, according to my invention, a cathode assembly structure comprises a thin walled tubular support sleeve of polygonal transverse section over which a thicker walled cylindrically tubular cathode sleeve is telescoped. The transverse dimension between the pairs of corners spaced the greatest distance apart of the polygon-shaped tubular support sleeve is greater than the inside diameter of the cylindrically tubular cathode sleeve, thus providing a pressure or tight frictional fit. The support sleeve is preferably provided with rounded longitudinal edges and cylindrically convex sides. This insures that as the two sleeves are telescoped together the support sleeve will be desirably distorted toward a cylindrical shape. Thus, the resulting contact between the two sleeves comprises at least a plurality of uniformly circumferentially spaced longitudinally extending elongated areas in contact through a small are rather than a spot or line contact.

According to my invention these elongated areas approach, and in the extreme case may consist of, a continuous single cylindrical area of contact. Such a cathode assembly structure according to my invention insures that the contact between the inner support sleeve and the outer emissive cathode sleeve is relatively extensive and positive.

In the drawings:

FIG. 1 is a longitudinal section of an electron tube incorporating my invention;

FIG. 2 is a transverse section view taken along line 22 of FIG. 1;

FIG. 3 is an enlarged longitudinal section of the cathode according to my invention as incorporated in the electron tube of FIGS. 1 and 2;

FIG. 4 is a transverse section view taken along line 4-4 of FIG. 3;

FIG. 5 is a side elevation view of the inner support sleeve of the cathode structure of FIGS. 3 and 4;

FIG. 6 is a transverse section view taken along line 66 of FIG. 5; and

FIGS. 7 and 8 are side and end elevation views respectively of apparatus suitable for forming the support sleeve of FIGS. 5 and 6 according to my invention.

In FIGS. 1 and 2, I show an electron tube 10 incorporating my invention. The tube 10 includes a ceramic disk header 12 having a plurality of bores 14 therethrough. A plurality of electrode support conductors 15 and lead-in conductors 16 are sealed in vacuum-tight relation in the bores 14, the walls of which have been metallized.

The electrode mount assembly comprises coaxial cylindrical anode and grid electrodes 26 and 28 and cathode electrode assembly 36, respectively. The anode 26 is mounted on a radially extending flange 3%, which is in turn mounted on one lead-in conductor 16 and two support conductors 15. The grid electrode 28 is similarly mounted on a radially extending flange 34 which is in turn mounted on one lead-in conductor 16 and two support conductors 15. The cathode assembly 30 includes a tubular cathode support sleeve 36 mounted on a radially extended flange 88, which is supported on one lead-in conductor 16 and two support conductors .15. The cathode assembly 30 also includes a cup-like tubular emissive cathode sleeve 40 which is disposed over the support sleeve 36, sleeve 40 being coated with a suitable electron emissive material 41.

Acoiled heater 44 is disposed in the cathode support sleeve 36 and connects to a pair of lead-in conductors 16 which are sealed through the header 12. As previously stated, all of the above-described elements except the cathode sleeve are all assembled in a jig in loose contacting relationships and brazed together at high temperatures into a strain-free rigid mount assembly. A vacuum-tight envelope is provided by a cup-shaped shell 46 sealed to the periphery of the ceramic disk header 12. The shell 46 includes a pair of longitudinal extending arcuate positioning lugs or tongues 47 and 48 which serve to protect the externally extending conductors '16 and facilitate socketing of the tube.

As previously stated the presence of the emissive coating 4-1 incorporation of the outer cathode sleeve 40 precludes into the tube prior to the brazing step. Accordingly, only the support sleeve 36 is included as a part of the brazed assembly. After this brazed assembly is removed from the brazing oven, the cathode sleeve 40 with its emissive coating 41 thereon is telescoped over the support sleeve 36 in a pressure fit therewith. The envelope is then sealed to the header after bake-out and exhaust as described above.

This sealing step is performed by an oven heating step, at a temperature below the temperature involved in the prior brazing step. The sealing step is performed simultaneously with an exhaust and bake-out of the tube. During the exhaust, bake-out, and sealing of the shell 46 to the ceramic header 12, a sufficiently high temperature is reached to sinter the support and cathode sleeves 36 and 46 together in a secure, permanently bonded assembly.

Referring to FIGS. 3 and 4, the cathode assembly structure 36 made according to my invention, includes emissive cathode sleeve 46 closed at one end so that when the two sleeves 36 and 40 are telescoped together, a bottoming of the support sleeve 36 within the emissive sleeve cup 40 will provide an automatic longitudinal indexing of the telescoped arrangement. In order to facilitate a telescoping of the two sleeves 36 and 40, one end 50- of the support sleeve =36 is provided with a tapered reduced diameter and an interned rim 52. This feature of the support sleeve 36, according to my invention, will be more fully hereinafter described with reference to FIG. 5.

FIGS. 5 and 6 illustrate the novel shape of the cathode support sleeve 36 made according to my invention. As shown in FIGS. 5 and 6 the unt-elescoped support sleeve 36 has over the major portion of its length a cross-section of generally polygonal shape. In the preferred embodiment the polygonal shape is hexagonal. Over a minor portion 50 of the length of the support sleeve 36, its cross-section merges from the hexagonal shape as shown in FIG. 6 to a cylindrical shape at the very end of the sleeve. This minor length over which the merging occurs is the tapered portion 50 as illustrated in FIG. 3.

In accordance with one feature of my invention, the polygonal cross-section of the support sleeve 36 is characterized by rounded longitudinal edges or corners 54 and slightly cylindrical convex sides 56. The greatest transverse dimension of the support sleeve 36, i.e., the dimension between any two opposite corners 54, is made to be slightly larger than the internal diameter of the cathode sleeve 40.

Because of this size relationship, when the hexagonal support sleeve 36 and the cathode sleeve 40 are telescoped together a pressure fit results. As telescoping is begun, the cathode sleeve is placed over the tapered end 50 of the hexagonal support sleeve 36 and the sleeves contact each other at the longitudinal edges or corners 54. Then, as the two sleeves are further telescoped together, the support sleeve 36 is distorted toward a more nearly cylindrical cross-section conforming to the internal cylindrical shape of the cathode sleeve 40. The support sleeve 36 is relatively thin walled compared with the cathode sleeve 40. This insures that the support sleeve will be distorted during telescoping to conform to the cylindrical shape of the cathode sleeve 40 rather than vice versa.

The generally polygonal shape of the support sleeve 36 insures that it will firmly contact the cathode sleeve 40 at least in a plurality of uniformly circumferentially spaced longitudinally elongated areas. Furthermore, the original rounded-edge and convex-side shape of the support sleeve 36 insures that a distortion of the support sleeve toward a cylindrical shape rather than an inward collapse thereof will result. This distortion results in a contact between the support and cathode sleeve 36 and 40 which approaches, and in the extreme case, may amount to, an essentially single area continuous cylindrical contact.

Distortion of the support sleeve 36 from its original hexagonal shape toward a cylindrical shape results in the support sleeve being stressed to a point somewhere below its elastic limit. Because the stressing of the support sleeve is below the elastic limit, the sleeve possesses a spring-like quality which tends to urge its original edge portionsradially outward. This urging results in a maintained radially acting pressurized contact between the two sleeves 36 and 40. This pressurized contact insures a continued tight frictional fit between the cathode sleeve 40 and its support sleeve 36.

Because of my invention, wherein a polygonal support sleeve 36 is employed, the variations within manufacturing tolerances of the two sleeves 36 and 40 will never result in a loose telescopic fit. The spring-like characteristic of the polygonal support sleeve 36 according to my invention results in the sleeve having a selfadjusting size feature. Thus, in either the extreme case wherein the cathode sleeve 40 is a maximum and the support sleeve 36 is a minimum, or the extreme case wherein the cathode sleeve 40 is a minimum and the support sleeve is a maximum, relatively large area, high pressure contact is assured. The distortability of the polygonal support sleeve according to my invention accommodates for such size relationship variations; the spring action of the sleeve insures that a tight frictional pressurized telescoped fit is maintained. Such accommodation and pressure fit is, .of course, not possible with the prior art assembly wherein two relatively undistortable cylinders are telescoped together.

The nature of the distortion of the polygonal support sleeve 36 toward conformity with the cylindrical cathode sleeve 4tl'is representatively illustrated by a comparison of FIGS. 4 and 6. FIG. 6 shows the support sleeve 36 before telescopic engagement with the cathode sleeve 4 to be generally hexagonal with rounded edges 54 and slightly cylindrically convex sides 56. FIG. 4 shows the support sleeve 36 in telescopic interference fitting engagement with the cylindrical cathode sleeve 40. It can be seen that the support sleeve 36 is distorted from its original generally hexagonal shape to a nearly cylindrical shape. The telescoped support sleeve 36 contacts the cathode sleeve all in areas extending circumferentially from six points '60 which correspond to the original six edges 54 of the polygonal sleeve 36. By virtue of the distortion of the support sleeve 36 toward cylindricality, the areas of contact 60' are more extensive circumferentially than would exist between the undistorted polygonal support sleeve 36 and a circumscribing cylinder which would not distort the sleeve. It can also be seen that the telescopically engaged support sleeve 36 is spaced from the cylindrical emissive sleeve 40 at points 62 which correspond to the original six sides 56 of the polygonal sleeve 36. Thus, good contact between the cathode sleeve support and the cathode is always assured.

As illustrated in FIGS. 4, 5, and 6, the cathode support sleeve 36 comprises a lap seam tubing member. The lap seam is indicated by the numeral 64. Although a lap seam 64 does slightly disturb the cylindrical continuity of such a tubular member and, furthermore, although a lap seam 64 results in a thickened wall section of the tub-ular member, I have found that such a seam apparently does not affect the practice of my invention. Accordingly, I have found it unnecessary to provide any particular angular orientation of the lap seam 64 with respect to the polygonal support sleeve 36.

I have further found that while the principle of my invention may be embodied in polygonal sleeves other than hexagonal, i.e., pentagons, heptagons, or octagons, that a hexagonal shape is preferred. A polygon of seven or more sides too nearly approaches a cylindrical shape in its undistorted condition to permit adequate distortion when it is changed from its polygonal to a near cylindrical shape. On the other hand, a polygon of five or less sides effects too great a distortion when it is changed from its polygonal to a near cylindrical shape. It will, of course, be appreciated that this factor does not depend upon the relative size of the support sleeve but wholly upon the number of sides of the polygonal cross-section thereof.

FIGS. 7 and 8 illustrate apparatus suitable for forming a polygonal tubular support sleeve 36 according to my invention from cylindrical tubular stock. A mandrel 66 comprises a rounded edge polygonal section 68 and a cylindrical end section 70 which merge together over a tapered section '72.

According to a preferred method of fabrication, a cylindrical tubular member is forced over the mandrel 66 and rolled to shape the tubing to the shape of the mandrel. I have found that tubing sized to fit with interference contact within the tubular cathode sleeve 4% to be of suitable size. Before the rolling action, the polygonal sleeve as is provided with its interned rim '52 and tapered end section 5!} as illustrated in FIG. 3.

I claim:

1. A cathode assembly comprising an outer cylindrically tubular cathode member and an inner tubular support member mounted within said outer tubular member in contacting telescoped relationship therewith, said inner tubular member being distorted from an untelescoped generally polygonal cross-section toward a cylindrical crosssection.

2. A cathode assembly comprising a first tubular support member having a generally polygonal cross-section and a second tubular cathode member having a cylindrical cross-section, said second tubular member being telescoped over said first tubular member in pressurized contact therewith to apply external forces to said first tubular member along the longitudinal edges of said support member to thereby stress and distort said first tubular member from its said generally polygonal cross-section toward a cylindrical cross-section.

3. A cathode assembly comprising a first tubular support member having a generally polygonal cross-section when in an undistorted condition and a second tubular cathode member having a cylindrical cross-section, said second tubular member being telescoped over said first tubular member in frictional contact therewith to distort said first tubular member from its undistorted generally polygonal cross-section toward a cylindrical cross-section.

4. A cathode assembly structure for an electron tube comprising a tubular support sleeve of generally polygonal cross-section and a cylindrically tubular electron emissive cathode sleeve having an inside diameter less than the greatest outside transverse dimension of said support sleeve, said emissive cathode sleeve being telescoped over said support sleeve to thereby distort said support sleeve toward a cylindrical shape whereby said sleeves contact each other at least in a plurality of substantially uniformly circumferentially spaced longitudinally extending elon gated areas.

5. A cathode assembly comprising outer and inner tubular sleeves in pressurized telescoped contact, said outer sleeve having a relatively undistortable cylindrical cross-section, said inner sleeve having a relatively distortable cross-section distortably stressed :below its elastic limit from a generally hexagonal cross-section having rounded longitudinal edges and cylindrically convex sides toward a cylindrical cross-section.

6. A cathode assembly according to claim 5 and where in said inner sleeve comprises part of a brazed assembly and serves as a cathode support sleeve and said outer sleeve has an electron emissive coating on the external cylindrical surface thereof.

7. A cathode assembly comprising an inner tubular support sleeve and an outer cylindrically tubular emissive sleeve telescoped together in frictional contact, said support sleeve being stressed below its elastic limit from an unstressed generally polygonal cross-section with rounded longitudinal edges, whereby said support sleeve presses radially outwardly against said emissive sleeve in a plurality of longitudinally extending, uniformly circumferentially spaced areas.

8. A cathode assembly comprising a tubular cathode sleeve, electron emitting material thereon, a tubular support sleeve for said cathode received within said cathode sleeve and having a transverse section tended toward that of a polygon, the longitudinal portions of said support sleeve defining the corners of said polygon being arcuate shaped, the portions of said support sleeve between said corners being bowed outwardly, said cathode sleeve being in contact with and exerting radial pressure on said corners to maintain said cathode support in compression.

9. A cathode assembly comprising a tubular cathode sleeve, electron emitting material thereon, a tubular sup port sleeve for said cathode sleeve received within said cathode sleeve and having a non-circular transverse section approaching that of a polygon, the longitudinal portions of said support sleeve defining the corners of said polygon being arcuately shaped, the portions of said support sleeve between said corners being bowed outwardly, said cathode sleeve being in contact with and exerting radial pressure on said corners to put said support sleeve under stress, said cathode sleeve and said support sleeve being sintered together in the areas of contact.

References Cited in the file of this patent UNITED STATES PATENTS 1,828,524 Delaney Oct. 20, 1931 2,524,001 Spencer Sept. 26, 1950 2,619,706 Vause Dec. 2, 1952 2,708,249 Pryslak May 10, 1955 

1. A CATHODE ASSEMBLY COMPRISING AN OUTER CYLINDRICALLY TUBULAR CATHODE MEMBER AND AN INNER TUBULAR SUPPORT MEMBER MOUNTED WITHIN SAID OUTER TUBULAR MEMBER IN CONTACTING TELESCOPED RELATIONSHIP THEREWITH, SAID INNER TUBULAR MEMBER BEING DISTORTED FROM AN UNTELESCOPED GENERALLY POLYGONAL CROSS-SECTION TOWARD A CYLINDRICAL CROSSSECTION. 